1. Field of the Invention
The present invention relates to side-light type spread illuminating apparatuses including a light guide plate that has a light incident end face on which a light source is placed and that emits planar illumination light from its emitting portion.
2. Description of Related Art
As illumination units of liquid crystal display (LCD) panels, sidelight type spread illuminating apparatuses (backlights) are widely used primarily in the field of small mobile information equipment such as mobile phones. In the side-light type spread illuminating apparatuses, a light-emitting diode (LED) having a small size and high environmental adaptability is placed along a side end face of a light guide plate. In these days, light guide plates are used in order to deal with further reduction in thickness of the small mobile information equipment. The light guide plate is a component in which a light incident wedge portion whose thickness is gradually reduced as away from the side end face having the LED placed thereon (hereinafter also referred to as the light incident surface) is formed between the light incident surface and an emitting portion. The use of such a light guide plate can reduce the thickness of the emitting portion of the light guide plate regardless of the thickness of the LED (see, e.g., Japanese Patent Application Publication Nos. 2007-287550 (JP 2007-287550 A) and 2008-170739 (JP 2008-170739 A).
In such spread illuminating apparatuses, a peripheral edge part of the emitting portion (including the light incident wedge portion and a part of the emitting portion which is located closer to the light incident wedge portion) serves as a non-light-emitting region by covering with a light shielding sheet etc. The emitting portion excluding the non-light-emitting region serves as a light-emitting portion, and light emitted from the light-emitting portion is used as illumination light.
In the case where the light guide plate has such a light incident wedge portion, a larger part of light entering the light guide plate through the light incident surface is emitted from the tilted surface of the light incident wedge portion to the outside and becomes leakage light in the non-light-emitting region. Moreover, the light that is incident on the emitting surface of the emitting portion after being reflected by the tilted surface is incident at a relatively small incident angle on a region of the emitting surface which is located closer to the light incident wedge portion. A large amount of light is therefore emitted from the part of the emitting portion which is located closer to the light incident wedge portion (normally, this light includes leakage light from the non-light-emitting region and emitted light from a part of the light-emitting region which is located closer to the light incident wedge portion). The leakage light from the non-light-emitting region reduces utilization efficiency of light emitted from the LED, and the emitted light from the part of the light-emitting region which is located closer to the light incident wedge portion reduces uniformity (evenness) of illumination light emitted from the light-emitting region. For example, JP 2007-287550 A proposes a solution to the above problem by setting the angle between the tilted surface and the emitting surface to a predetermined angle or less.
Recently, with reduction in thickness of spread illuminating apparatuses, there has been an increasing demand to reduce the size of the non-light-emitting region (i.e., to reduce the frame width) according to an increase in size of the liquid crystal display region. In order to satisfy this demand, it is necessary to reduce as much as possible the length in the light guide direction of the light incident wedge portion (the length of a tilted portion) that cannot be used as the light-emitting region.
However, in order to reduce the length in the light guide direction of the light incident wedge portion on the condition that the light incident wedge portion has a given maximum thickness (the thickness of the light incident surface, corresponding to the dimension in the lateral direction) and a given minimum thickness (corresponding to the thickness of the emitting portion), it is necessary to increase the angle between the titled surface of the light incident wedge portion and the emitting surface. However, increasing the angle between the tilted surface and the emitting surface increases the amount of light that is emitted from the tilted surface to the outside and becomes leakage light from the non-light-emitting region and the amount of light that is emitted from the part of the emitting portion which is located closer to the light incident wedge portion and becomes leakage light from the non-light-emitting region or emitted light from the part of the light-emitting region which is located closer to the light incident wedge portion. This reduces utilization efficiency of light emitted from the LED and uniformity (evenness) of illumination light emitted from the light-emitting region.
Accordingly, in such spread illuminating apparatuses including a light guide plate having a light incident wedge portion, a design idea is required which reduces the size of the non-light-emitting region as much as possible on the condition that maintains high light utilization efficiency and high uniformity of illumination light.
In relation with this, the applicant of the present invention found that the critical vale capable of reducing the length in the light guide direction of the light incident wedge portion while maintaining high light utilization efficiency and high uniformity of illumination light was approximately 1.78 times the maximum thickness of the light incident wedge portion regardless of the thickness of the emitting portion, and proposed a spread illuminating apparatus in which the length in the light guide direction of the light incident wedge portion was set to approximately 1.78 times the maximum thickness of the light incident wedge portion (see Japanese Patent Application Publication No. 2011-96522 (JP 2011-96522 A)). According to the spread illuminating apparatus described in JP 2011-96522 A, the length in the light guide direction of the light incident wedge portion can be reduced as much as possible and the size of the light-emitting region of the spread illuminating apparatus can be increased as much as possible while maintaining high light utilization efficiency and high uniformity of illumination light.
However, the demand to reduce the thickness of the LCD devices and to increase the size of their display regions is becoming increasingly strict, and it is strongly desired to further reduce the thickness and the frame width of the spread illuminating apparatuses used as a backlight of the LCD devices, while maintaining high light utilization efficiency (and high luminance of illumination light) and high uniformity of illumination light.